This invention is directed to improvements in composite self lubricating bearings. More particularly, this invention provides improvements to bearings which have been sold under the denomination of "DU" bearings. Such bearings are well known to those skilled in the art as comprising a metal backing layer; a porous layer, generally of sintered bronze, on the metal backing layer; and a cold-formed, sintered layer on the porous layer of a self-lubricating material. The self-lubricating material has traditionally comprised polytetrafluoroethylene and lead in intimate admixture. The bearings are distinct from heat processed bearings such as those which employ thermal extrusion or injection molding.
Such composite bearings, while enjoying widespread utility in shock absorbers, gear pumps, MacPherson struts, and a wide variety of other applications, have suffered from significant shortcomings and limitations. In particular, these bearings have exhibited a tendency towards cavitation, that is, to the formation of erosion sites in the self-lubricating layer leading to cavities, delamination, stripping, and general wear. These shortcomings have been exacerbated by the tendency towards employing such bearings in shock absorbers and MacPherson struts which require increased load bearing and sliding capabilities. Moreover, such bearings must withstand hydraulic stress caused by exposure to turbulent oil flow present in connection with many bearing applications. Thus, a long-felt need has arisen for bearing materials which exhibit decreased cavitation, which avoid stripping of bearing materials from the sliding surface of the bearing, and which exhibit improved wear resistance and concomitant longer life. This long-felt need is reflected in the attempts of others to solve the foregoing problems.
U.S. Pat. No. 3,882,030--Campbell is directed to bearings which are compression or injection moldable. Campbell employs polyphenylene sulfide in large amounts as a carrier resin for the injection molding process. The bearings are not generally based on polyfluorinated material.
U.S. Pat. No. 3,994,814--Cairns, assigned to the assignee of this invention, is directed to injection molding and melt extrusion production methods for the manufacture of bearings. The processes thus disclosed are intended to "avoid costly cold forming and sintering techniques" and include major proportions of thermoplastic resins to permit such processing to take place. The thermoplastic resins may include polyarylene sulfides.
U.S. Pat. No. 4,000,982--Ueda is directed to certain improvements in cold-formed bearings through the inclusion of zinc metal in lieu of the traditionally used lead. This patent is significant for its recognition, as of its 1975 filing date, of the wear problems experienced in reciprocating devices such as shock absorbers and the like.
U.S. Pat. No. 4,172,622--Baker et al. reduces friction in certain composite-structure bearings through use of predominant amounts of polyphenylene sulfide. PTFE and other components may be added to the PPS.
U.S. Pat. No. 4,115,283--Needham is directed to anti-friction compositions for bearings comprising, predominantly, polyarylene sulfides. Solid lubricants, including PTFE, may be admixed in minor proportion. Fillers, including hydrated aluminum silicates, may also be included. Reinforcing materials such as carbon, glass, boron, or other fibers are an obligatory constituent. The materials are adapted for employment in injection molding or other thermoplastic molding processes.
U.S. Pat. No. 4,139,576--Yoshimura is directed to compositions useful for thin coatings on cookware to provide non-stick properties and the like. The coatings comprise polyarylene sulfide resin, imido resin, and a fluorocarbon polymer.
U.S. Pat. No. 4,312,772--Mori provides certain improvements in composite bearing materials and reflects the long felt need for additional improvements. The patent discloses certain lead compounds and alloys in the lubricating layer of bearings.
U.S. Pat. No. 4,394,275--Bickle et al. provides bearing materials including from 30 to 60% polyvinylidene fluoride to improve machinabality.
U.S. Pat. No. 4,157,273--Brady is directed to the employment of polyarylene sulfide combined with PTFE as an adhesive. Polyarylene sulfide is present in proportions of at least 2:1 over the PTFE in the compositions. The reference teaches away from the employment of these materials in bearings.
U.S. Pat. No. 4,439,484--Mori discloses the wearing and cavitation problems which are experienced with self-lubricating bearings. Mori attempts to overcome these problems by using PFA/PTFE low-friction coatings on composite backings.
Bearings have been sold under the name "Clean Metal" by the Senju Company of Japan. They are believed to be composite bearings comprising, as a lubricating layer, a PTFE-acetal copolymer together with polyphenylene sulfide and lead. This bearing is believed to have been formed through an extrusion process, requiring a high proportion of acetal polymer or the use of an acetal co-polymer sufficient to allow the material to melt and flow.
The present invention overcomes the shortcomings of prior composite bearings or diminishes their effect by providing improved composite bearings employing a synergistic combination of materials for the sintered, self-lubricating layer. The combination has certain critical limitations, as will more fully hereinafter be described.